During operation, a gas turbine engine compresses intake air, mixes the compressed air with fuel, and ignites the fuel-air mixture to produce combustive gasses, which are then expanded through a number of air turbines to drive rotation of the turbine rotors and produce power. Turbine nozzles are commonly positioned upstream of the turbine rotors to meter combustive gas flow, while also accelerating and turning the gas flow toward the rotor blades. A turbine nozzle typically assumes the form of a generally annular structure having a number of flow passages extending axially and tangentially therethrough. By common design, the turbine nozzle includes an inner ring (also commonly referred to as an “inner endwall” or “inner shroud”), which is generally annular in shape and which is circumscribed by an outer ring (commonly referred to as “outer endwall” or “outer shroud”). A series of circumferentially-spaced airfoils or vanes extends between the inner and outer rings. Each pair of adjacent turbine nozzle vanes cooperates with the inner and outer rings to define a different combustive gas flow path through the turbine nozzle. Flanges are provided around the inner and outer rings to permit attachment of the turbine nozzle to the static engine infrastructure utilizing, for example, bolts or other fasteners.
Turbine nozzles have traditionally been produced as relatively large, unitary castings. More recently, turbine nozzles are often assembled from multiple, separately-cast pieces with each cast piece including an arched segment of the inner ring, an arched segment of the outer ring, and a number of vanes (e.g., one to five vanes) extending between the inner and outer ring segments (commonly referred to as a “segmented turbine nozzle”). In either case, the turbine nozzle vanes are typically fixed between the attachment points of the turbine nozzle (e.g., flanges extending radially from the inner and outer rings); and little, if any, radial compliancy is provided between the vanes and the locations at which the turbine nozzle is affixed to the engine infrastructure. For this reason, conventionally-produced turbine nozzles are typically unable to accommodate disparities in thermal growth between the turbine nozzle vanes and the other portions of the turbine nozzle occurring during engine operation as the vanes are bathed in combustive gas flow and significant thermal gradients develop across the body of the turbine nozzle. High stress concentrations may thus occur at the interfaces between the turbine nozzle vanes and rings, which can result in undesirably rapid thermomechanical fatigue and a reduced operational lifespan of the turbine nozzle.
It has recently been recognized that thermomechanical fatigue can be greatly reduced by imparting a turbine nozzle with slip joints, which permit relative radial movement between the turbine nozzle vanes and one or more of the turbine nozzle attachment points. U.S. Pat. No. 8,047,771 B2, entitled “TURBINE NOZZLES AND METHODS OF MANUFACTURING THE SAME,” issued Nov. 1, 2011, and assigned to the assignee of the present application, describes turbine nozzles having cast annular bodies to which separately-produced flanges are joined. A slip joint is created between the cast annular body and at least one of the annular flanges to permit relative radial movement between the turbine nozzle vanes and at least one of the locations at which the turbine nozzle is mounted to the static engine infrastructure, which reduces thermomechanical fatigue of the turbine nozzle over its operational lifespan. This advantage notwithstanding, turbine nozzles having slip joints of this type remain limited in certain respects. For example, the slip joint design of such turbine nozzles can place undesired constraints on airfoil or vane geometry. Furthermore, as the turbine nozzle vanes and rings are produced as a single cast body, feature complexity within casting (e.g., the complexity of vane cooling flow passages) can be limited, scrap quantities can be relatively high due to voiding or other defects occurring within the larger castings, and the usage of single crystal superalloys in the fabrication of the vanes may be precluded.
It is thus be desirable to provide embodiments of a method for fabricating turbine nozzles including slip joints, which overcome most, if not all, of the above-noted drawbacks. It would also be desirable to provide embodiments of a turbine nozzle produced in accordance with such a fabrication method. Other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description and the appended Claims, taken in conjunction with the accompanying Drawings and the foregoing Background.